This run is an opportunity to run two of the city’s nicest parks and also enjoy some important sites. The Cinquantenaire is a lovely park with beautiful manicured paths. It is home to the Arcades du Cinquantenaire, sort of a meld of the Arc de Triomphe and the Brandenberg gate. There are also several museums: Royal Museum of the Armed Forces and Military History, Art and Architecture, Auto Musuem, and the Islamic Center. It is ~1,7 miles around the perimeter. For a longer run, take Rue de Loi (N3a), passing several EC buildings, east to Royal Park. This park is a bit smaller, and lies between the royal palace and the Belgian parliament. At the southeast corner is a Museum of Musical Instruments and Modern Art Museum. And just east of Royal Park is Gare Central and the heart of Brussels’ museums and tourist areas.
Note: A nice add-on to this run is Leopold Park.